Underground storage tank (UST) and above-ground storage tank (AST) integrity testing has become a lucrative market, driven partly by regulatory requirements and partly by sound environmental protection policy. Various systems and methods for leak testing of storage tanks and pipelines, often used for petroleum crude or refined-product storage and transport, have been introduced to meet this need. One method to detect leaks in these vessels and pipelines involves adding a specialty compound or mixture (called a "tracer") to the product being stored or moved that is both soluble in the product and not ordinarily present in the product or in the environment. Subsequent detection of this tracer compound or mixture outside the vessel or pipeline system can demonstrate that the tracer mixture has escaped the system, thereby indicating the system has developed a leak.
A typical tracer release detection application involves blending a tracer such as sulfur hexafluoride (SF.sub.6) (a nontoxic, inert gas) with petroleum-related products in a pipeline or storage tank. Halogenated nonpolar compounds, halogenated methanes, halogenated ethanes, halogenated propanes and propenes, halogenated butanes, cyclobutanes and butenes have also been used as tracers to test fuel storage and pipeline systems. Tracer compounds have also been used to locate the underground presence and/or movement of water, soil gases, petroleum, or natural gas. Tracers have also been used to help define the presence and continuity of geologic faults and permeable formations. In each case, the specialty compound or mixture, soluble in the phase or medium of interest and not ordinarily present in the environment, is introduced at a particular location. Successful sampling for the tracer at points removed from the original release point then indicates "communication" with or "continuity" to the original release point.
Conventional tracer release detection methods to detect fuel leaks involve analyzing soil vapors drawn from sampling wells surrounding the fuel storage system for evidence of the tracer escaping the storage system. Typically, companies using tracer-related test methods locate sampling wells in the soil adjacent to fuel storage equipment. A background sample is usually taken prior to introducing the tracer compound into the fuel storage system to provide a baseline for the soil surrounding the storage tank prior to the actual tracer-related test.
After installing sampling wells and taking background tracer measurements, a technician will typically introduce one or more tracer chemicals (in either gas or liquid phase) into the fuel storage system. A predetermined mass of tracer(s) is inserted into the fuel storage system through a single tubing line inserted into the storage tank. An alternative tracer introduction system can involve placing an enclosed gas-permeable membrane containing a given mass of tracer(s) into the storage tank and having the tracer release through the membrane over a period of time. If a storage system has a leak, the tracer may escape the storage system with the fuel.
After some time period has elapsed, a technician uses a vacuum pump attached either to the top of the sampling well, or to tubing placed into the well, to draw soil vapors through the bottom end of the sampling well into a sample container. Typically, some portion of the soil vapor sample in the container is injected into a gas chromatograph equipped with an electron capture detector (ECD) to analyze the vapor sample for the presence of the tracer. These test samples are then compared to the background samples to determine if a product leak exists.
These conventional tracer release leak detection systems have several limitations.
For storage tank testing by tracer techniques, the desired physical properties of tracer compounds include high volatility. A compound's boiling point is often a good indicator of volatility. For example, SF.sub.6 boils at about -50.7.degree. C., making it relatively volatile at ambient temperatures. However, the more volatile a tracer compound, the harder it is to keep that compound near a sampling point, in a sample container, or in an analytical system such as a gas chromatograph, an autosampler or a gas sampling loop/injector. Volatile tracers require care to insure the integrity of sample containers and analytical system components. Conventional tracer release detection systems avoid using tracers more volatile than SF.sub.6 for these reasons (and some systems use tracers even less volatile than SF.sub.6), sacrificing analytical sensitivity and volatility for a decreased risk of accidental or cross-contamination.
Conventional tracer release detection methods using a soil vapor sampling system do not always collect adequate masses of tracer from the tracer-affected soil vapor space near or in the sampling well to detect relatively small amounts of tracer released from a leaking storage system. These conventional systems only pull the soil vapor sample from the relatively small volume opening at the bottom end of the sampling well. Thus, relatively small leaks may go undetected in the soil vapor sampling tracer detection systems. This problem can be exacerbated by low-permeability soil conditions surrounding the sampling wells. This problem can also limit the ability to accurately identify the location of a leak.
Conventional tracer release detection systems must also find a way to exclude or remove water from the vapor sample when using halogenated hydrocarbons or other electronegative compounds (such as sulfur hexafluoride) as a tracer. This must be done because an electron capture detector has relatively high sensitivity to water. The steps taken in conventional systems to exclude or remove water from the sample can potentially degrade the quality and/or the quantity of the sample.
Conventional soil vapor sampling tracer release detection systems using a vacuum pump and a container to hold the extracted soil vapors typically pull soil vapor samples from a sampling well for two to twenty minutes. The system is therefore limited to acquiring tracer sample that is present within, or in very close proximity to, the sampling well air space during that relatively short time period. The lack of a concentrating mechanism in these systems can allow the tracer to migrate away from the sampling well and be missed during the sampling process. This "short-term" sampling process can result in missing tracer releases because the tracer has 1) already declined below detectable concentrations within and near the sampling well, or 2) has migrated away due to subsurface soil conditions, weather conditions or other factors.
Conventional soil vapor sampling tracer release detection methods sample soil vapor by processes that result in a smaller mass of tracer collected. A low concentration accidental contamination of a sample has a higher likelihood of producing a "false positive" test result with a conventional, lower-mass tracer sample than with a higher mass tracer sample as can be taken using the present invention.
Conventional tracer release detection methods use a single delivery tube to introduce tracer into the fuel. As a result, the tracer disperses less rapidly and less uniformly throughout the fuel. This can lead to fuel leaking out that contains a less than optimal concentration of the tracer. This, in turn, can increase the difficulty of detecting the tracer, and ultimately, the leak.
Conventional tracer testing methods present a problem if a leak is at the bottom of a tank that has collected water at the bottom of the tank. While the tracers currently used have high solubility in fuel, they have low solubility in water, and therefore, a leak may go undetected.